Water coming from under bell... cause?

I just turned on the sprinkler system for the year (I'm in Minnesota) and have a problem I have not encountered in the seven or eight years we have had this system. With the water line open, water is coming out at a pretty rapid rate from under the "bell" (not sure if my terminology is correct). Other than that, system is working fine. Can someone help me as to the cause for this, and what I can do to fix it? Thank you very much.

(If it helps, I have a company come and winterize but turn it on myself in the spring.)

Sometimes is you open the bottom ball valve (shut off No.1) slowly the valve inside will stick a little.

Turn the bottom ball valve off, wait for a few seconds, then turn it on very fast. Normally you would want to turn all water valves on and off slowly to prevent water hammer. This type of valve, on this type of backflow assembly (a pressure vacuum breaker (PVB)) has to be turned on fast to seal the rubber seat inside the bell. The fast opening of the ball valve will smack the water against the seat and should push the seal closed and stop the water flow.

Thanks, Mick. I tried it and no go. (I am assuming that by the bottom ball valve you mean theone by the arrow in my photo.) Will continue trying various water on/off combinations. But if that does not work, what are my other options? Thanks again.

Also, I have never had the PVB tested. My bad? Sprinkler service folks have never mentioned it. Do I need to have them come out and do it?

Yep that is the one that I was talking about. I think that it's time to get someone in to get it serviced. How long have you had this assembly? Check with your municipality. This (PVB) should be tested every year and the test sheet should be sent to the municipality or water district. Different municipalities have different rules. I have heard stories of every year, every two years, and every five years. The manufacturer recommends every years, as do most water districts. The cost is anywhere between $35.00 to $125.00 per test.

Thanks, all. I think we have had this seven or eight years. I tried to pop the canopy off, but while the nut turns the bolt or whatever is inside it turns as well so it won't come off. I guess time to have the pros come out and service it.